1. Field of the Invention
The present invention relates to a zoom lens system and an electronic imaging apparatus provided with a zoom lens system.
2. Description of Related Art
Zoom lens systems having four lens groups configured of a positive first lens group, a negative second lens group, a positive third lens group and a positive fourth lens group, in that order from the object side, are known in the art (e.g., Japanese Unexamined Patent Publication Nos. 2002-6215, 2008-70450 and 2002-162564) as zoom lens systems that aim to achieve further miniaturization and a higher optical quality.
Such zoom lens systems having four lens groups employ an internal focusing system which predominantly uses the second lens group to perform zooming, uses the third lens group to compensate for fluctuations in the imaging plane position that occur during zooming, and divides the first lens group into a front sub-lens group and a rear sub-lens group and performs focusing this rear sub-lens group. According to such a configuration, the length of the entire optical system (zoom lens system) does not change during zooming or during focusing operations, so that an optical system (zoom lens system) in which the f-number fluctuates minimally can be achieved.
However, in the zoom lens system taught in Japanese Unexamined Patent Publication No. 2002-6215, since the number of lens elements provided in each lens group (especially in the fourth lens group) are too few, fluctuation in lateral chromatic aberration increases during a focusing operation. Furthermore, in regard to the zoom lens system taught in Japanese Unexamined Patent Publication Nos. 2008-70450 and 2002-162564, since part of the fourth lens group is used as an image-blur correction lens group (image-stabilizing lens group) that is moved in a direction orthogonal to the optical axis, the number of lens elements thereof is large, complicating the configuration and increasing costs.
Furthermore, in each of the zoom lens systems taught in the above-mentioned Japanese Unexamined Patent Publication Nos. 2002-6215, 2008-70450 and 2002-162564, the refractive-power balance between the front sub-lens group and rear sub-lens group (focusing lens group) of the first lens group is inadequate. Accordingly, in the above-mentioned Japanese Unexamined Patent Publication Nos. 2002-6215 and 2008-70450, the refractive power of the rear sub-lens group (focusing lens group) of the first lens group becomes too strong, so that large amounts of various aberrations such as spherical aberration and astigmatism occur, especially at the long focal length extremity. Furthermore, in regard to the zoom lens system taught in above-mentioned Japanese Unexamined Patent Publication No. 2002-162564, the refractive power of the rear sub-lens group (focusing lens group) of the first lens group is too weak, so that due to the amount of movement of the focusing lens group being large, fluctuation in chromatic aberration increase during a focusing operation.